Sub-surface radar imaging

ABSTRACT

Detailed is radar imaging apparatus including a single transmit antenna and at least one receive antenna, scanning apparatus (e.g. a pantograph) for mechanically scanning the antennas over a surface of interest, position providing apparatus (e.g. a computer driving the pantograph via an X-Y drive and a stepper motor) providing a position signal indicative of the instantaneous position of the transceiver and a control system for operating the transmit antenna in a stepped frequency continuous wave mode. The amplitude and phase components of the receive antenna signal are analysed and the output combined with the position signal as in a synthetic aperture array to provide a radar image signal of the surface and underlying features. The scan is two-dimensional (random or boustrophedral).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International ApplicationNo. PCT/GB02/04181 filed on Sep. 13, 2002 and published in English asInternational Publication No. WO 03/025613 A2 on Mar. 27, 2003, whichapplication claims priority to Great Britain Application No. 0122357.7filed on Sep. 15, 2001, the contents of which are incorporated byreference herein.

The present invention relates to imaging of features lying below asurface such as a wall using radar techniques.

While radar was originally developed for the detection and tracking ofremote objects from a base station which may be stationary, as in aflight control centre, or moving, as in a ship, aircraft or othervehicle, it has found other uses, and among these is the imaging ofsurface and sub-surface features in a variety of applications.

One such application is the investigation of ground features. Thus U.S.Pat. No. 5,325,095 (Vadnais) refers to prior art using an airborneplatform and describes a ground penetrating radar (GPR) system usefulfor detecting buried objects such as mineral deposits, undergroundrivers and caverns, and underground artefacts such as buried ordnance,waste, storage tanks, pipes, sewers, cables, etc. Phase quadraturesignals are transmitted, and the received signals are demodulated toprovide a DC signal for digitisation and conversion into the frequencydomain by a fast Fourier transform. The resulting signal providesinformation about the range and cross-section of a target.

Another ground penetrating bistatic radar system for similar purposes isdescribed in U.S. Pat. No. 5,499,029 (Bashforth), the value andmagnitude of a preponderant frequency in the resulting signal beingindicative of target range and signature.

Both of these disclosures show a single transmit and a single receiveantenna (a bistatic radar system), and neither refer specifically to thescanning of the radar apparatus over an area larger than theinstantaneous coverage.

However, where the surface is relatively extensive, the radar apparatuscan be effectively scanned over the surface, for example by beingmounted in a vehicle or aircraft. In such a case it is necessary toknow, or to be able to derive, the position of the radar apparatus forcorrelation with the radar signal, in order to be able to derive a mapof the entire area. For example, U.S. Pat. No. 5,673,050 (Moussally)describes a 3-dimensional radar imaging system for the detection andmapping of underground objects and voids, in which a singletransmit/receive antenna is operated from a moving vehicle such as ahelicopter, and the radar signals are combined with position signalsfrom a GPS or inertial navigation system in a synthetic aperture radar(SAR) technique. Moussally employs an interrupted frequency modulatedcontinuous wave signal where the transmitted signal changes frequencycontinuously, whereas the first aspect of the present invention employsa stepped frequency continuous wave providing a quasi-static system.

Similarly, U.S. Pat. No. 5,796,363 (Mast) discloses a SAR system inwhich position is derived by triangulation from signals from threereference radar time-of-flight measurement units, and provides thefacility to look at structures comprised of layers of dissimilarmaterials that effect the radar pulse propagation velocity differently,one example being asphalt over concrete and rebar. This uses a singletransmit antenna and a single receive antenna.

International Patent Application No. WO 98/58275 (Forsvarets) relates toa monostatic pulsed mode radar for imaging at relatively long range.

Surface penetrating radar imaging systems may be implemented withrelatively long wavelength radiation. The latter feature enables thesurface to be efficiently irradiated using a remote transceiver, but theresolution is relatively low. The use of shorter wavelength radiation isnot so practical or efficient using remote irradiation, due torelatively rapid attenuation of the signal. Thus, where the surface tobe investigated and/or the features to be detected may be of arelatively small scale, techniques where the radar antennae arerelatively remote may not be appropriate.

U.S. Pat. No. 5,030,956 (Murphy) discloses a radar tomography system formedical imaging in which the antenna is associated with a standarddental X-ray cone 0.5 metres long.

U.S. Pat. No. 5,835,053 (Davis) shows a roadway ground penetratingsystem to provide measures of the depth and thickness of pavementstructure layers, having top and bottom interfaces. The radar apparatusis mounted in a towed trailer and includes an array of at least tworeceive antennae differently spaced from a transmit antenna.

The timings of the signals reflected from the interfaces are combinedwith the known receive antennae spacing to provide a measure of signalvelocity, from which can be derived layer depths and thicknesses. SAR isnot mentioned.

U.S. Pat. No. 5,835,054 (Warhus) also discloses a system having radarapparatus including a receive/transmit antenna array mounted to a truck,e.g. on the bumper. In this case, the radar return amplitude and timesignal information is combined with X-Y co-ordinate position informationto provide 1, 2 or 3 dimensional imaging information. Radar phaseinformation is not obtained or used, and the radar is a pulsed system,not a continuous wave system.

U.S. Pat. No. 5,357,253 (Van Etten) discloses an earth probing systemusing a tuned transmit antenna for operating at a relatively lowfrequency. The latter requires switching circuitry whereas inembodiments of the present invention untuned wideband transmit andreceive antennas are employed. The radar is operated in the frequencydomain, but SAR processing and coherent integration of data are notmentioned.

U.S. Pat. No. 5,900,833 and International Patent Application No.97/41449 (both Sunlin) also disclose the use of a moving array oftransmitting and receiving antennae which is moved, for example on atank or helicopter, for use in a SAR material penetrating imagingsystem. It is said to be suitable for detecting small objects close to asurface by using very narrow pulses, and also for the detection of largedeep objects by using wider pulses. Phase information is not obtained orused.

European Patent Application No. 0 744 629 (Hughes Missiles) discloses animaging radar for providing images of objects behind obscuringstructures such as concrete and stucco barriers and walls, or withinbuildings. This uses a transmit antenna and an array or receiveantennae, and operates on time and amplitude of reflected signals. Nomention is made of the use of phase information, or of theimplementation of SAR techniques.

U.S. Pat. No. 5,969,661 (Benjamin) uses a phased array of transmittingantennae for irradiating a selected voxel within a search volume. Theuse of focussing is also the subject of an article“Synthetically-Focussed Surface-Penetrating Radar for Operation From aMoving Vehicle”, R Benjamin et al, 2^(nd) EUREL International Conferenceon the Detection of Abandoned Lancimines, Edinburgh, October 1998.

The EG&G Silverrod imaging system employs a 2 to 6 GHz stepped frequencyradar in which a 60 cm. square area is scanned to produce maps of 16depth planes. Bistatic log-periodic antennae are mounted side by sideand moved over the area in any manner, with X-Y co-ordinates beingderived from an ultrasonic time of arrival location system.

The present invention facilitates the implementation of a highresolution short range radar imaging method in which a radar transceiveris moved over a surface in relatively close proximity thereto, forproviding images of features lying in or under the surface. In the rangeof frequencies appropriate for sub-surface imaging of this type,normally in the range 1 to 20 GHz, it is necessary to use a transceiverlocated relatively close to the surface, and preferably within 2 or 3wavelengths of the surface for efficient operation.

The surface to be scanned may be a wall or other vertical surface.However, it may also be any other surface such as the ground or a groundfeature such as a road or pavement, or a roof of a building.

The apparatus provided by the present invention may be useful in anumber of areas of interest, including the detection of unexplodedordnance or other explosive devices, for example in demining an area ofground; in ground clearance where it is necessary to be able to detectthe presence of, for example, dumped materials and hazardous waste; intransport, for example for assessing roads and runways; and in civilengineering, for example in bridge and building testing, looking forerosion in structures, as in building bars; and for the internalexamination of containers such as suitcases.

In a first aspect the present invention provides radar apparatuscomprising a transceiver including a single transmit antenna and atleast one receive antenna, scanning means for mechanically scanning saidtransceiver over a surface of interest, position providing meansproviding a position signal indicative of the instantaneous position ofthe transceiver, control means for operating the transmit antenna in astepped frequency continuous wave mode, signal analysing means foranalysing amplitude and phase components of the receive antenna signal,and signal combining means for combining the output of said signalanalysing means with said position signal as in a synthetic aperturearray (SAR) to provide a radar image signal of the surface andunderlying features. The first aspect extends to a related method ofimaging sub-surface features.

The stepped frequency continuous wave mode is a process in which a CWsignal is transmitted and received for a period prior to incrementingthe frequency, and repeating the process a number of times. In theembodiment the frequency steps are obtained by the use of a staircasevoltage generator coupled to a voltage controlled oscillator. Employmentof this mode as opposed to impulse radar permits a wide bandwidth to beused with relatively simple hardware, and avoids any requirement forfast accurate sampling circuits. It is preferred that the frequencyrange used be as wide as possible, since in general this increases theavailable resolution of the image. Thus in use of the invention thebandwidth of the radar frequencies employed is preferably at least 8 GHz(for example in the range 8 to 16 GHz), more preferably at least 15 GHz(for example in the range 5 to 20 GHz), and even more preferably atleast 20 GHz (for example in the range 1 to 20 GHz). The bandwidth ispreferably centred within the range 8 to 16 GHz, more preferably withinthe range 9 to 14 GHz, and even more preferably within the range 10 to13 GHz (as exemplified in the ranges above the centre frequencies are12, 12.5 and 11.5 GHz respectively).

In the first aspect, the invention enables the provision of a SARsub-surface image of (for example) a wall with improved resolution andfaster scanning than prior art systems.

At frequencies in the 1 to 20 GHz range, the time difference between theradiated and return signals is so short that it is difficult to providegating which is sufficiently fast to enable separation of the twosignals if a monostatic transceiver (single antenna acting for bothsignals) is provided. Hence the present invention uses at least oneseparate receive antenna (bistatic arrangement). In a preferredembodiment, a single receive antenna is provided. An advantage of thebistatic arrangement is that it permits imaging at both short and longranges. The provision of one or more receivers enables both the alignedand orthogonal polarisations to be exploited in the signal processing(c.f. the arrangement in Moussally, which uses a single linearlypolarised antenna exploiting the Brewster angle).

Furthermore, at these frequencies, sampling of the return signal isparticularly difficult (at up to 40 GHz) when a pulsed system isused—the rise and fall times of sampling pulses for use in such systemswould normally be measured in picoseconds. Therefore, the apparatus ofthe present invention is preferably arranged to be used in a continuouswave mode, which eases the sampling requirements significantly.

The phase and amplitude information may be embodied in I and Q signals(in-phase and quadrature signals), e.g. from mixing the receive antennasignal with a reference signal. These signals may be further processedto provide the amplitude information as separate phase quadratureamplitude signals I, Q as a function of time (i.e. real and imaginarytime domain data), for example by use of an inverse fast Fouriertransform circuit (hence the use of different frequencies in the steppedcontinuous wave mode, which may be regarded as facilitating the depthresolution, returns from the same feature having different phase andamplitudes depending on the frequency).

The signal combining means includes means for performing the SARfunction in known manner. If position of the transceiver and a voxel isspecified, the distance and hence time delay of the signal may becalculated. On the basis of this delay) the I and Q signal amplitudes atthe corresponding delay may be extracted from the I and Q time domaindata for that voxel, and by suitably combining (coherently adding) suchinformation from receive antenna signals for antenna positions over thewhole scanned area, pairs of voxel I and Q signal amplitudes may bederived for each and every voxel of the imaged volume associated withthe scanned surface (a voxel being an elementary unit volume of an arrayconstituting the imaged volume).

For each voxel, the signal combining means may be arranged to determinethe square root of the sum of the squares of the I and Q components ofthe voxel I and Q signal amplitudes to provide a voxel total amplitudesignal. The array of voxel total amplitude signal provides said radarimage signal, which may then be displayed.

The array of voxels thus provided is relatively coarse, so giving riseto an image of relatively low resolution. Thus it is preferred, as inthe embodiment, for the signal combining means to be arranged tointerpolate the voxel I and Q signal amplitudes for all the voxels areover a finer resolution array of voxels, each voxel of said finer arraythereby having separate interpolated quadrature signal amplitudes, priorto the squaring and adding step, i.e. the latter is preferably performedon interpolated voxel quadrature signal amplitudes to provide voxeltotal amplitude signals for a finer array.

The scanning means enables the scanning of the surface of interest withmultiple passes, as in boustrophedral scanning for example. Thus thescanning means enables a 2-dimensional scan of the surface, ensuringuniform illumination in both dimensions, thereby giving better signalgain in the SAR processing with equal lateral resolution in bothdirections and better image quality. This is to be contrasted with muchof the prior art listed albove which employs SAR techniques, but in a“spotlight mode” where data is collected after a single pass over thetarget.

The scanning means may comprise a pantograph on which the transceiver ismounted. As particularly described, the pantograph is operativelycoupled to an X-Y scanner, which in turn is coupled to a computer viastepper motors for precise and accurate control. The computer may alsoact as the position providing means.

In a second aspect the invention provides radar apparatus for theinvestigation of sub-surface features, the apparatus comprising a radartransceiver mounted on a pantograph for scanning over a surface ofinterest, and also extends to a related method of scanning a radartransceiver.

In the second aspect, the invention enables the mechanical scanning of aradar transceiver over a surface such as a wall in close proximitythereto, for example for providing a sub-surface image of featurestherein. The use of a pantograph is advantageous in that the transceiverportion of the apparatus can be collapsed down to provide a compactportable apparatus. It also enables the scan position to be accuratelycontrolled, e.g. to within around 1 mm in the embodiment to bedescribed, while maintaining a relatively fast scanning motion.

As particularly described, the pantograph is driven by stepper motorscontrolling an X-Y scanning frame under the control of a computer whichalso provides a position signal for use in data processing. However, itwould equally be possible to provide a position sensing arrangement onthe transceiver, or that end of the pantograph for providing anindependent position signal.

Particularly where the surfaces to be encountered are likely to beuneven, or where there is a possibility that the transceiver to surfacespacing may alter, for example due to deformation of the pantographunder gravity or in a windy situation, maintaining means may be providedfor maintaining the transceiver a predetermined distance away from thesurface. For example, a wheel may be provided on the transceivermounting, or on the pantograph in the vicinity of the transceiver forcontacting the surface. Where the surface is a horizontal upward facingsurface such as a floor, this may be all that is necessary with reliancebeing placed on gravity to maintain the wheel to surface contact. Whereother effects come into play, biasing means may be provided, e.g. aspring, for biasing the wheel or other maintaining means into contactwith the surface.

The apparatus according to the second aspect may frtther includeposition providing means for providing a position signal indicative ofthe instantaneous position of the transceiver relative to a referencepoint on the pantograph. This function may be effected by the aforesaidcomputer.

Means may be provided for combining output signals from the transceiverwith the position signal to provide a synthetic aperture array signal,which may then be processed to develop an image signal, e.g. fordisplay.

As in the first aspect the transceiver may be operated operating in thefrequency range 1 to 20 GHz, preferably in a continuous wave mode, andmore preferably in a stepped frequency mode.

The transceiver of the second aspect may comprise separate transmit andreceive antennae, and preferably a single transmit antenna and/or asingle receive antenna.

In a preferred embodiment according to either aspect, the number of armsof said pantograph is variable for altering the area of said surfacewhich is scanned.

Further advantages and features of the invention may be derived from aconsideration of the appended claims, to which the reader is referred,and also from a reading of the following more detailed description of anembodiment of the invention, made with reference to the accompanyingdrawings, in which:

FIG. 1 is an outline schematic block diagram of a portion of anapparatus according to the invention, including stepped continuous waveprocessing circuitry and a transceiver portion;

FIG. 2 is an outline schematic block diagram of an image signalprocessing portion of the apparatus according to the invention,complementary to the portion shown in FIG. 1; and

FIG. 3 shows in schematic form a transceiver positioning system usefulfor the apparatus of FIGS. 1 and 2.

FIG. 1 schematically shows a surface 16 of interest including aconcealed object 17 for scanning by a transducer arrangement 29 coupledto stepped continuous wave processing circuitry 30. The transducerarrangement 29, which is shown in more detail in FIG. 3, includes hornantennae 4 and 5 which are fixed relative to each other and can bescanned over the surface 16.

The stepped continuous wave processing circuitry 30 includes a staircasevoltage generator 1 controlling a frequency controlled oscillator (VCO)2, which is coupled via an amplifier 3 for driving a single transmitradar antenna or horn 4. Returned radiation is received by one or morehorn antennae 5, the output therefrom being fed via an amplifier 6 and asplitter 7 to RF mixers 8, 9. Mixer 9 receives a reference signaldirectly from the VCO 2 for in-phase demodulation, whereas mixer 8receives the same signal via a 90° phase shifter 10 for quadraturedemodulation. The outputs of mixers 8, 9 are coupled via respective lowpass filters 11, 12 to respective sample and hold circuits 13, 14capable of storing N values at any time. The values held in circuits 13,14 are coupled to an N point inverse fast Fourier transfer (FFT)processor 15 providing real and imaginary data outputs 27, 28 fortransmission to a synthetic aperture array (SAR) processing circuitshown in FIG. 2.

As shown in FIG. 2, a circuit 20 coupled to a computer controlling thescanning of the antennae 4, 5 provides antenna position data X, Y, andalso outputs 3-coordinate voxel position data, i.e. data P, Q, Rdefining an elementary volume in the volume being inspected by theapparatus. Respective circuits 18 and 19 receive the data 27, 28 whereit is combined with the output of circuit 20 in a manner known per se.Preferably this step is accomplished by analysing the data 16, 17 forall values of P, Q, R before altering the values of X and Y. However,other known techniques for SAR analysis may be employed.

The outputs of the circuits 18, 19 are fed to respective circuits 21, 22where they are integrated over all values of X and Y in respect of eachvoxel. The signals derived from the sampling process 13, 14 arerelatively coarse, and if relied upon per se would result in relativelypoor quality images. Hence it is preferred that circuits 21, 22interpolate, the signals from circuits 18, 19, for example by a linearinterpolation, prior to the integration process.

Up to this point the I and Q type signal have been processed separately.Subsequently, for each voxel the real output from the circuit 21 issquared at 23 and the imaginary output from the circuit 22 is squared at24. At 25 the outputs of circuits 23 and 24 are added and the squareroot of the resulting sum is determined to provide a voxel value 26which can then be displayed at a corresponding position, e.g. on atwo-dimensional display 35 arranged to display one plane of voxels 36 ata time.

In use, the transducer arrangement 29 enables the horns 4, 5 to bemechanically scanned over a surface of interest such as a wall, road orpavement, and in close proximity thereto. As shown in more detail inFIG. 3, for this purpose the horns are mounted at or adjacent one end ofa pantograph 31. The arms 32 at the other end of the pantograph 31 aresecured to a pair of movable members 33 mechanically coupled to acomputer controlled X-Y scanning frame 34 driven by stepper motors. Thespacing of members 33 may be altered to control the extension of thepantograph and the position of the antennae 4, 5 in the Y direction, andthe pair of members 33 may be moved laterally to control the position ofthe pantograph and antennae 4, 5 in the X direction.

In use, the transducer arrangement 29 enables the horns 4, 5 to bemechanically scanned over a surface of interest such as a wall, road orpavement, and in close proximity thereto. As shown in more detail inFIG. 3, for this purpose the horns are mounted at or adjacent one end ofa pantograph 31. The arms 32 at the other end of the pantograph 31 aresecured to a pair of movable members 33 mechanically coupled to acomputer controlled X-Y scanning frame 34 driven by stepper motors. Thespacing of members 33 may be altered to control the extension of thepantograph and the position of the antennae 4, 5 in the Y direction, andthe pair of members 33 may be moved laterally to control the position ofthe pantograph and antennae 4, 5 in the X direction.

The sizing is such that an area of around 50 cm square may be scannedaccording to any desired pattern as determined by computer control.However, the coverage may be varied by altering the number of arms inthe pantograph, or by providing an arrangement 29 with a differentsizing.

In use, a preferred scanning pattern is boustrophedral, but any type ofscan, including a random scan, could be used. If a low resolution imageis initially acceptable, this could be generated from an initialrelatively coarse scan, with additional information from further scannedlocations in a refinement of the overall scan then being obtained toimprove the image resolution.

Although the embodiment shows a single transmit antenna and a singlereceive antenna, it is possible to provide further transmit and/orreceive antennae to increase the amount of data and to improve thesignal to clutter ratio, particularly if polarisation effects areexploited. However, this may require modifications of the apparatus, forexample the pantograph may need to be strengthened or otherwise adaptedto support the additional weight, and unless a plurality of steppedfrequency sources and receivers are provided, it will be necessary toprovide switch multiplexers, which may be difficult at the preferredfrequencies.

For example, if a linear polarisation transmitter is used, a singleantenna could be used to receive the co-polarisation orcross-polarisation signal, or a pair of receive antennae may be providedfor receiving both of these signals. Similarly if a transmit antennaemits circularly polarised radiation, a single antenna could be used toreceive a signal of one linear polarisation, or a pair of receiveantennae may be provided for receiving signals with orthogonally linearpolarisations. In either case, the signal from the additional receiveantenna generally facilitates and enhances detection and analysis ofsub-surface anomalies. The transmit antenna may be capable ofselectively emitting linear and circular polarised radiation.

1. Radar imaging apparatus comprising a transceiver including a singletransmit antenna and at least one separate receive antenna, controlmeans for operating the transmit antenna in a stepped frequencycontinuous wave mode, scanning means for mechanically scanning saidtransceiver across a surface of interest, position providing meansproviding a position signal indicative of the instantaneous position ofthe transceiver, signal analysing means for analysing amplitude andphase components of the receive antenna signal, and signal combiningmeans for combining the output of said signal analysing means with saidposition signal as in a synthetic aperture array to provide a radarimage signal of the surface and underlying features, characterised inthat (a) the apparatus is arranged for operation over a bandwidth of atleast 8 GHz; and (b) part of the scanning means in use is located at afixed position relative to said surface and the position providing meansis arranged to provide a position signal as a function of theinstantaneous position of the transceiver relative to the said fixedposition.
 2. Apparatus according to claim 1 wherein said scanning meansis arranged for scanning over said surface in two dimensions. 3.Apparatus according to claim 1 wherein said scanning means comprises ascissor linkage.
 4. Apparatus according to claim 3 wherein the number ofarms of said scissor linkage is variable for altering the area of saidsurface which is scanned.
 5. Apparatus according to claim 3 wherein saidscissor linkage is coupled between an X-Y scanner and the transceiver.6. Apparatus according to claim 5 wherein said X-Y scanner is coupled toa computer for control thereof.
 7. Apparatus according to claim 6wherein said computer is said position providing means.
 8. Apparatusaccording to claim 1 wherein said signal analysing means is arranged toconvert said receive antenna signal into separate phase quadratureamplitude signals I, Q.
 9. Apparatus according to claim 8 wherein saidsignal analysing means is arranged to transform each of said signals I,Q into the time domain.
 10. Apparatus according to claim 8 wherein saidsignal combining means is arranged to correlate separate said signals I,Q from the signal analysing means with voxel locations and said positionsignal for determining respective pairs of voxel I, Q signal amplitudesfor each said voxel.
 11. Apparatus according to claim 10 wherein foreach voxel said signal combining means is arranged to determine thesquare root of the sum of the squares of the respective voxel I, Qsignal amplitudes to provide a voxel total amplitude signal. 12.Apparatus according to claim 10 wherein said signal combining means isarranged to interpolate said voxel I, Q signal amplitudes for all thevoxels over a finer resolution array of voxels, each voxel of said finerarray thereby having separate interpolated I, Q signal amplitudes. 13.Apparatus according to claim 12 wherein for each voxel of said finerarray said signal combining means is arranged to determine the squareroot of the sum of the squares of the respective interpolated voxel I, Qsignal amplitudes to provide a voxel total amplitude signal. 14.Apparatus according to claim 11 wherein the array of voxel totalamplitude signal provides said radar image signal.
 15. Apparatusaccording to claim 1 and including means for displaying said radar imagesignal.
 16. Apparatus according to claim 1 wherein said single transmitantenna is for transmitting a linear and/or circular polarisationsignal.
 17. Apparatus according to claim 1 comprising a single receiveantenna for receiving a linear and/or circular polarisation signal. 18.Apparatus according to claim 1 comprising at least two receive antennaefor receiving respective signals with different polarisations. 19.Apparatus according to claim 1 wherein said bandwidth is centred withinthe range 8 to 16 GHz.
 20. A method of imaging sub-surface featurescomprising the steps of providing a transceiver including a singletransmit radar antenna and at least one separate receive radar antenna,operating the transmit antenna in a stepped frequency continuous wavemode to irradiate a surface of interest, providing a scanner formechanically scanning the transceiver across said surface, providing aposition signal indicative of the instantaneous position of thetransceiver relative to the stationary part of the scanning means,analysing the phase and amplitude of the receive antenna signal andcombining the resulting information with said position signal as in asynthetic aperture array to provide a radar image signal of the surfaceand underlying features, characterised in that (a) the apparatus isoperated over a bandwidth of at least 8 GHz; and (b) part of thescanning means in use is maintained at a fixed position relative to asurface of interest and the position signal is determined as a functionof the instantaneous position of the transceiver relative to the saidfixed position.
 21. A method according to claim 20 wherein saidmechanical scanning is performed across the surface in two dimensions.22. A method according to claim 20 wherein said scanning is effected bydriving a scissor linkage on which said transceiver is mounted.
 23. Amethod according to claim 22 including the step of adjusting the numberof arms of said scissor linkage for altering the area of said surfacewhich is scanned.
 24. A method according to claim 22 wherein saidscissor linkage is driven by controlling an X-Y scanner operativelycoupled thereto.
 25. A method according to claim 24 wherein said X-Yscanner is controlled by a computer.
 26. A method according to claim 25wherein said providing a signal indicative of the instantaneous positionof the transceiver effected by said computer.
 27. A method according toclaim 20 wherein said step of analysing includes deriving from saidreceive antenna signal separate phase quadrature amplitude signals I, Q.28. A method according to claim 27 wherein said step of analysingincludes signal transforming each of said signals I, Q into the timedomain.
 29. A method according to claim 27 wherein said combining stepincludes correlating said signals I, Q from the signal analysing meanswith voxel locations and said position signal for determining respectivepairs of voxel I, Q signal amplitudes for each said voxel.
 30. A methodaccording to claim 29 wherein for each voxel the respective voxel I, Qsignal amplitudes are squared, and added and the resulting sum is squarerooted to provide a voxel total amplitude signal.
 31. A method accordingto claim 29 wherein said voxel I, Q signal amplitudes for all the voxelsare interpolated over a finer resolution array of voxels, each voxel ofsaid finer array thereby having separate interpolated I, Q signalamplitudes.
 32. A method according to claim 31 wherein for each voxel ofsaid finer array the respective interpolated voxel I, Q signalamplitudes are squared, and added and the resulting sum is square rootedto provide a voxel total amplitude signal.
 33. A method according toclaim 30 wherein the array of voxel total amplitude signals providessaid radar image signal.
 34. A method according to claim 20 andincluding the step of displaying said radar image signal.
 35. A methodaccording to claim 20 wherein a single receive antenna is provided. 36.A method according to claim 20 wherein said bandwidth is centred withinthe range 8 to 16 GHz.